1. Field of the Invention
The present invention relates to a lifting device having parallel double screw rods, and in particular to a lifting device mounted in the electric table or electric bed.
2. Description of Prior Art
Conventionally, a lifting device is provided within a two-stage sliding unit and then connected to an electric table or electric bed, so that the higher or lower position of the electric table or electric bed can be adjusted. Since the total length of the extension of the lifting device does not exceed the sum of two sliding units and the total length of the retraction is not lower than the length of any one longer sliding unit, the extension and retraction stroke that is possibly achieved is limited and thus cannot satisfy the requirement for a larger adjusting space in the existing market. Therefore, there is proposed a multiple-stage (such as three-stage or four-stage) sliding unit. In addition to have a larger amount of extension and a shorter amount of retraction than those of the two-stage sliding unit, the moving speed of extension or retraction of the multiple-stage sliding unit is also faster. Thus, it is an important issue for those skilled in this art to provide a connecting structure having a lifting device and a multiple-stage sliding unit.
In a conventional lifting device having a screw rod, as disclosed in EP1230872, the three-stage lifting device comprises a driving means, a first transmission means and a second transmission means. The first transmission means comprises a cylindrical screw rod and a nut driven by the driving means. The inside and outside of the cylindrical screw rod are provided with inner and outer threads, respectively. The outer thread is threadedly connected to the nut. The second transmission means and the first transmission means are provided in series. The second transmission means has a pillared screw rod threadedly connected with the inner thread of the cylindrical screw rod, so that the cylindrical screw rod can move linearly relative to the nut and the cylindrical screw rod at the same time. Further, with the transmission of the driving means, each sliding unit can slide relative to each other.
However, in practical use, the conventional lifting device of screw rod has still some problems as follows. Since the primary members such as the cylindrical screw rod and the pillared screw rod are manufactured separately, those elements cannot be interchangeable, so that the total cost of the elements cannot be reduced efficiently. Further, the inside and outside of the cylindrical screw rod are provided with threads, the machining process is very complicated and difficult. Also, after the machining process, it is not easy to perform the heat treatment to obtain a good surface hardness. As a result, it is very fast to generate wear between the cylindrical screw rod and the pillared screw rod, causing the increase in the subsequent cost for maintenance and inconvenience in use.
In view of the above, the inventor proposes the present invention to overcome the above problems based on his expert experiences and deliberate researches.